Component 09 of 10 · Vision & compliance

Glazing

Fire-rated vision panel. The transparent barrier.

A vision panel lets people see what they are opening a door into. It is also a hole cut in the barrier. Fire-rated glazing uses specialist glass, intumescent-lined beads and strict limits on size and position, all fixed by the fire door's evidence.

Key facts

Function: Provides vision through the door without breaking its rating
Specification: Glass type, aperture size and position all limited by the fire door's evidence. Beads, seals and fixings as the tested glazing system specifies
Materials: Wired, ceramic or intumescent laminated fire glass, beads to the system specification, intumescent glazing seal each side of the pane
Critical tolerances: Beads, seals and fixings exactly as the tested system. Bead pins commonly at about 150 mm centres, per the system's instructions. No movement in the pane
Standards: Glazing systems proven within the fire door's test evidence. Where the pane must also be safety glass, its impact performance is classified to BS EN 12600
Role in fire: Maintains integrity across the aperture and, with the right glass, insulation too
Where it sits: Cut and glazed by a licensed processor within the scope of the door's evidence, never freehand on site

Compliance

Cutting a vision panel on site takes the door outside its evidence. Apertures must be cut and glazed by a licensed processor working within the scope of the door's evidence.

Did you know

Modern intumescent-laminated glass turns opaque and swells in fire: the interlayers foam up to form an insulating shield you can no longer see through.

On the model

Fire glass: Stamped with its rating
Glazing bead: As the tested system specifies
Glazing seal: Between glass and bead
Fixing pins: Beads pinned as the system specifies

What inspectors check

Work through the checklist with the door closed, then in motion. Every line here is a recordable outcome for a Fire Door Inspector.

Check the beads against the glazing system's specification, not against habit
Look along every bead for splits, damage and missing pins
Press the pane: it should not move in the aperture
Look over the glass for cracks, chips and deep scratches
Look for the glass mark, and check the type against the evidence
Look for signs the aperture was cut on site rather than by the processor
Check the intumescent glazing seal is present and not painted solid

Common defects

Beads not the type the glazing system specifies
Beads loose, split or damaged
Bead fixings missing, or too far apart
Pane loose or rattling in the aperture
Glass cracked, chipped or scratched
Glass not fire rated, or not the type in the evidence
Aperture cut on site
Intumescent glazing seal missing, damaged or painted over
Components mixed from different glazing systems

Go deeper

The glass itself

Glass does not burn, which is the reason people assume any glass will do. What matters is whether it stays in one piece and in the frame while everything around it is trying to make it fall out.

Ordinary float glass: Cracks and drops out within minutes of the heat reaching it, leaving an open hole through the barrier. It has no place in a fire door aperture.
Wired glass: The wire mesh holds the pane together after it has cracked, so the opening stays closed. It gives integrity, not insulation: the glass gets hot enough to ignite things touching the other side.
Fire rated ceramic: Clear, no wire, and stable at temperatures that would break ordinary glass. Also integrity rather than insulation unless combined into an insulating build-up.
Intumescent laminated: Interlayers between the panes foam and turn opaque under heat, blocking radiated heat as well as flame and smoke. This is what gives insulation as well as integrity, at more weight and more cost.
Integrity and insulation: E is integrity: the barrier holds and flame and hot gases do not get through. EI adds insulation: the unexposed face stays cool enough not to set fire to what is against it. A vision panel specified as E in a place that needed EI is a specification error, not an installation one.

It is a system, not a pane

The glass is one part. The beads, the fixings and the intumescent around the pane were all in the test together, and swapping any one of them means the result no longer describes what is in the door.

Beads: match the system, do not assume the species: Hardwood is the most common, and systems that specify it often set a minimum density too. But it is not the only answer: MDF beads are used, and some proven systems use softwood carrying an intumescent coating that does the work the density would otherwise do. What matters is that the bead is the one the system was tested with.So the question on site is not what species it is, it is whether it matches the system. A hardwood bead swapped into a system proven with coated softwood is as much a change as the reverse. Get the glazing system's data, not a rule of thumb.
Bead fixings: Steel pins or screws, long enough to hold into the leaf, and spaced closely. About 150 mm centres is typical, though the system's own instructions set the actual figure. Widely spaced fixings let the bead lift between them.
Intumescent glazing seal: A strip between the bead and the glass, on both faces, that expands and grips the pane as the timber around it retreats. Painted solid or left out, the pane loses what was meant to hold it once the beads start going.
Do not mix systems: Beads from one maker, seal from another and glass from a third is not a tested assembly, whatever each part is rated for on its own.

The aperture

Every vision panel is a hole cut in a fire barrier. Whether that hole is acceptable was decided before the door left the factory.

Cut and glazed within the scope of the evidence: Size, shape and position are all limited by the door's evidence, and the work is done by a processor licensed to do it. A door can carry a scope allowing an aperture that has not been cut yet, which is different from allowing any aperture at all.
Site cutting: Cutting a vision panel into a certified leaf on site takes the door outside its evidence. Nothing about the resulting door is evidenced any more, including the parts nobody touched.This is worth looking for rather than assuming. Signs are a panel that does not match others in the same run, tool marks, mismatched beads, filler around the opening, or an aperture in a position the manufacturer does not offer.
Damage: Cracks, chips and deep scratches all weaken a pane that has to survive thermal shock. Damaged glass in a fire door is not cosmetic.

Fire Door Anatomy is an introduction to fire doors, not the whole subject. Always defer to the fire door’s certification, its test evidence and a competent inspection.